Electric spark gap



Sept. `14, 1948. R. w. LAMPHr-:RE 2,449,397 j ELECTRIC VSPARK GAP FiledJan. 31, 1945 Patented Sept. 14, 1948 N, unirsi) STATES PATE i `ELECTRICSPARK GAP Richard W, Lamphere, Wlbraham Mass., as-

signor to American Bosch Corporation, Spring-, field, Mass., acorporation of New York Application January 31, 1945, Serial No. 575,418'c claims. (c1. 25o-ansi The present inventionrelates to electric sparkgaps, and particularly to those for oscillatory electric circuits, suchas have `been used in electric ignition systems of the capacity`discharge type for internal combustion engines, for` example,.as shown`in Fig. 1 of Patent 2,030,228 and also in lightningarresters, and inelectronic and other electrical devices.

Under the condition that the applied voltage across the gap is increasedfrom its initial value to the breakdown value` at an infinitely slowrate of l about l volts per second or less, the breakdown Voltagebetween two separated electrodes forming a spark gap in space,hereintermed a steady state breakdown voltage, dependson the gapdistance between the electrodesthe` composition and` density yof theatmosphere in thel gap, `the size and shape of the electrodes at the twopoints between which the spark occurs, andthe distribution of the boundelectrical` `charges in the space surrounding the gap. However, thevalue ofy the breakdown voltage `is materially affected by the steepnessor slope of the wave front and` by the shape of the wave front of theapplied voltage when it is increased in `value at a much higherratemeasured in kilovolts per micro-second and herein termed the transientbreakdown voltage. The charge, mass, cross sectional area, andgeometrical lposition of all free charged particles within the gas in asphere of` diameter having the two electrodes as poles, in addition tothe foregoing factors affecting the steady state breakdown voltage,affect the transient breakdown voltage, and likewise the work functionof the electrode material expressed in electron volts as represented bythe kinetic energy whichlan electron within `the material` must have,`in a direction normal to the surface of the electrode,

system to provide reliable ignition, but, as a result thereof, increasesthe altitudeat which the sys- Ina` temwill operate onaircraft"withoutelectrical :flash over. v

` `In such sparkgaps in general,` it has beencustcmary to assistinstabilizing or rendering uniform `the transientbreakdown voltage ofthegap by sealing `the spaced electrodesthereof in an:- envelope, usuallyof glass for insulating purposes,4 tovthereby reducevariations inperformance `due to surrounding atmospheric conditions resulting;` fromsuch variablesasaltitude, temperatureand humidity, and also by using`certain metals and alloys for `the electrodes with diierent gapseparation and with different gases and admixtures; thereof at diierent`pressuresin theenvelope. i In` i certain cases alsocer,tain activatingagents, and

certain shapes of'electrodes, with `different shape and size ofelectrode shields and envelopes therefor have :been used. `Certain ofsuch features are shown in Patent 2,122,932. Notwithstandingsuchfeatures, these spark gaps deteriorate and change in performance intheoperative `periodloetween overhauls'of the -devices or apparatus withwhich such spark gaps are associated;` particularly `in the case ,ofAspark1 gaps installed `in proximity to. grounded devices.` The groundeddevice lcauses distortion of the electric.fieldbetweenthe electrodesvforming the; gap,` and vdeflect the sparks, inthe gap -tothe insidewall of the envelope.`

This lowers the breakdown` voltage across the gap and rendersit unstableor erratic inperforrnance.V

Furthermore, the` sparks so `deiiected quickly cause ametallic depositon the inside oi the en-` i velope, lowering thebreakdown voltage stillmore.` with still further instability and erratic perform-i ance of thespark gap. i i

In accordance with my invention, the operation of spark gaps in generalinvarious kinds of` oscillatory circuits or` electrical devices is imoreor lessl stabilized throughoutv the `operative period` thereof whetherornot some or` all ofthe afore-l mentioned'featuresof prior spark gaps areused I accomplish Vthis bylzprovidingfmeans forlmaintaining asubstantially uniform voltage gradient i betweenthe two electrodes ofthe gap, irrespective of the proximity of outside grounded devices.. Thedeectionofthespark discharges is thereby` reducedor eliminated, and` thedistribution of` point.` electrical charges in thespaceoutsiolethe.`

envelope is stabilized or the variation `of their feffectisrend-erednegligible..` `Such means withinr my invention maycomprise a``conducting or, semi-conducting coating,` Aor a high resistance` suchlas a winding orlwotherwise electrically shunted across theelectrodes ofthe gap. "Preferably thiscoating or other resistance is arranged,`

outsidedzheA envelope, of the spark gap, whether along the outsidesurface thereof or` on a suit-` able suppprtspacedtherefrom, `so as tobeun-` NToFFICE affected by the heat or the chemical action from the sparkdischarges, Still more preferably the outside of the envelope iscompletely covered with a suitable semi-'conducting coating extending tothe outwardly projecting terminals of the electrodes forming the gap,and the envelope is symmetrical and coaxial with respect to the gap sothat the electric field is uniform and the voltage gradient between theelectrodes is substantially constant throughout. This condition is thenmaintained even though the potential of points outside of the envelope,and near thereto, are caused to assume values which otherwise woulddistort or deflect the field between the electrodes and thereby renderthe gradient nov longer constant. Also, preferably, good electricalcontact between this semi-conducting coating and the electrodes formingthe gap is made by a relatively high conducting coating which isarranged on the outside surface of the envelope around the terminalsprojecting therefrom, and which extends from both terminals to a placeoverlapping the semi-conducting coating to reliably connect the latterto both electrodes. Furthermore, these coatings are mechanicallyprotected by an outside layer of varnish, shellac, or the like, or by anadniixture of the semi-conducting material therein.

During the period of time that the voltage applied to the gap isincreasing, a displacement current ows between 'one electrode and thegrounded device, the other electrode of the spark gap being at groundpotential prior tothe breakdown of thegap. The magnitude of thisdisplacement .current is preferably made negligibly small, say %V orless 'as compared to the conduction current through the coating, so thatthe potential at all rpoints on the 'surface to which the coating isapplied will be determined almost entirely by the conducting coatingrather than by -f the grounded device in proximity to the gap.

- These and other objects and advantages `of my invention will appearfrom the following description of its preferred form in application, asan example, to a spark `gap for a capacity discharge type of ignitionsystem 'for an internal combustion engine `for aircraft. In thedrawings,

Fig. 1 is awce'ntral section, partly in elevation, of this preferredform of spark gap; and Fig. 2 is an end "elevati-on `of the `spark gapwith portions of the superposed coatings removed.

Referring to the drawings, the reference numeral I `designates `anyenvol-orne or container of uranium glass. Its tungsten electrodes 2 and3 have their proximate faces spaced about 3 m-m. apart to form a gap 4therebetween, and the tungsten rods of stems and 6 project in a sealedmanner through the envelope to form the outside terminals 'I and 8 ofthe spark gap. The shields `9 and I'D are of thin metal to assist inpreventing eroded electrode material from depositing on the envelope insuch manner as to form a leakage path in a shuntwith the gap, the wallsof which are arranged symmetrically and coaxially with respect to thegap 4 and lwith'the uppermost and lfowermost parts of the shields 9 andI0 atleast as distant as the axial length of the gap 4. Before thesealing 'tubulatio'n at I'I isl'rfus'ed off lto seal the envelope, andbefore the envelope Vis evacuated and then filled with argon, atleast98% pure, to an absolute pressure of V30 to 'i5 cms. of mercury, a fewdrops of radio-active solution, as a verydilute solution of Aradiumbromide in water, is introduced within the envelope. The order ofmagnitude of the'transient breakdown -vloltage of such a gap when new asused with an aircraft type of capacity discharge igni-` tion systems isaround '2000 crest volts. However, gaps of this gen-eral kind may bedesigned for a transient breakdown voltage of from 500 to 5000 crestvolts depending upon the size and type of the transformer used in thecircuit of the spark gap.

A spring connector I2 tightly gripping the rod 'I :of the spark gapfurnishes electrical connection to the electrode 2, and a suitableterminal (not shown) generally of a disconnectible type furnishesconnection to the rod 8 and therethrough to the electrode f3.

Both'ends of the spark gap, with the connector I2 installed as shown inthe drawings, are separately dipped into a highly conducting solutionsuch as a colloidal suspension of graphite in water. Thus the outside ofenvelope I around the end thereof and over terminal 8 is coated in acircular area, as shown at I3, with the Aquadag, and likelwise the otherend including the connector I 2. This provides a good electricalconnection between the respective rods 1, 8 and the surrounding endportions of the envelope. After the Aquadag has dried, the entire sparkgap with connector i2 is then immersed in a solution of semi-conductingmaterial Ifl, of a high resistance conductive coating material, overwhich is placed a coating of shellac. The highly conductive coatingreliably connects the semi-conducting coating to both terminals 'I and 8to form therewith a high resistance shunt electrically connected acrossthe yelectrodes 2, 3. After that semi-conducting coating has dried, thespark gap and connector I2 are given a coating of insulating varnish I5of heator air-drying type to protect the coatings underneath fromrubbing off or from so readily being damaged. The terminal area of rod 8is scraped or otherwise `cleaned off, as indicated in Fig. 1, to providegood electrical connection with the connector that may be installed atthat `end of the spark'gap.

It will be understood that many metal-oxide glazes adherent to glass,and suspensions of charcoal in varnish, may be used for thesemi-conducting coating with good results. So also sprayed metal orreduced platinum oxide of suit-v able kind may be used for thehighlyconducting coating around the terminals 'I and 8, and shellac ofsuitable kind may be used instead of the varnish. Furthermor'evthe highresistance coating maybe directly admixed with the outside protectivecoating so `that the two coatings are thus combined into one. However,the coatings as above specified are preferred at this time. Furthermore,it will be understood that other suitable materials and dimensions ofthe envelopes, gaps, electrodes, `stems and shields may be used, andlikewise 'other mixtures and pressures of gas within the envelope.

The resistance of the 'completed coating between the terminals 1, 8 canbe very high, ofthe order of 10 megohrns or more, and stili beadeduate'l'y conducting to maintain a reasonably uniform electric fieldacross the gap 4. The exact value ofthe maximum permissible resistancefor satisfactory operation depends in each case upon the geometry of thespark gap, the location of the nearby grounded bodies and the rate ofuse of `voltage applied to the gap, and can be predetermined in 'wellknown manner. With such uniform iield, or substantially so, the voltagegradient is substantially uniform between the electrodes 2, 3,irrespective of the location with respect to the spark gap of outsidegrounded devices, as for instance the grounded metallic radio-shieldingharness in lwhich the spark gap and other devices are contained. Thedeiiection of the rapidly-recurring spark discharges across the gap flis thereby reduced or eliminated, and the distribution of pointelectrical charges in the space outside the envelope l is stabilized ortheir effect on the spark discharges is rendered negligible. In this waylittle or no metallic deposit forms on the inside surface of theenvelope resulting from `erosion of the electrode or electrodes, andtherefore there occurs no substantial decrease in the electricalresistance of the'gap throughout its useful life such as would renderthe gap erratic or unstable. Accordingly, the transient breakdownvoltage ci the gap is maintained substantially constant, therebystabilizing or rendering more uniform the action of the electricdischarge in the circuit or devices with which the spark gap isassociated, and also reducing the voltage required in the system toprovide reliable ignition for aircraft engines while even operating athigh boost or lean mixture and at high temperatures of operation, andalso increasing the altitudes at which the system will operate withoutelectrical fiashover. Such conditions obtain throughout the entireoperating period between overhauls of 450 hours or more.

A 500 hour test on a large number of externally coated spark gaps asspeciiically described herein, and a substantially similar number ofuncoated but otherwise identical spark gaps, using about 2000 crestvolts at a thousand sparks per minute, showed for the coated spark gapsabout deviation in breakdown voltage against an average maximumdeviation of about for the uncoated spark gaps, and a reduction inconductivity from about one megohm resistance to a practically innitevalue of resistance for the coated gaps. Such deviation as occurred withthe uncoated gaps may result in the absence of ring at some spark plugsof aircraft engines.

It will be understood that many modifications and changes may be made inthe structures and use of the spark gaps herein disclosed withoutdeparting from the spirit of the invention as covered by the broad termsof the appended claims.

Having thus described the invention, what I claim is:

1. An electric spark gap comprising an envelope, electrodes mountedwithin the envelope, said electrodes being suiciently closely spaced toeach other to provide a spark gap, and a, semiconducting coatingexternally of and entirely enclosing said envelope, said coating beingsufficiently conductive to conduct bound charges, but not suiiicientlyconductive to short circuit the gap, said semi-conducting coating beingelectrically connected with at least one of said electrodes.

2. An electric spark gap comprising an envelope, electrodes mountedwithin the envelope, said electrodes being sufficiently closely spacedto each other to provide a spark gap, and a semiconducting coatingexternally of and entirely enclosing said envelope, said coating beingsuiciently conductive to conduct bound charges, but not sulcientlyconductive to short circuit the gap, and relatively higher condu-ctingmeans connesting said semi-conducting coating with at least one of saidelectrodes.

3. An electric spark gap comprising an envelope, electrodes mountedwithin the envelope, said electrodes being suiciently closely spaced toeach other to provide a spark gap, and a semi-conducting shuntexternally of and entirely enclosing said envelope, said shunt beingsufficiently conductive to conduct bound charges but not suiiicientlyconductive to short circuit the gap and relatively higher conductingmeans connecting said semi-conducting shunt with at least one of saidelectrodes.

4. An electric spark gap comprising an envelope, electrodes mountedwithin the envelope, said electrodes being sufficiently closely spacedto each other to provide a spark gap, and a resistor externally of andentirely enclosing said. envelope, said resistor being suiiicientlyconductive to conduct bound charges, but not sufficiently conductive toshort circuit the gap and relatively higher conducting means connectingsaid resistor with at least one of said electrodes.

5. An electric spark gap comprising an envelope ci insulating material,electrodes mounted within the envelope, said electrodes beingsufficiently closely spaced to each other to provide a spark gap, and asemi-conducting coating externally of and covering the entire surface ofsaid insulating material, said coating being suiciently conductive toconduct bound charges, but not suiiiciently conductive to short circuitthe gap, said semi-conducting coating being electrically connected withat least one of said electrodes.

6. An electri-c spark gap comprising an envelope of insulating material,electrodes mounted within said envelope, said electrodes beingsufficiently closely spaced to each other to provide a spark gap, and asemi-conducting coating externally of and extending over the entiresurface of said insulating material, said coating being suicientlyconductive to conduct bound charges, but not suiiiciently conductive toshort circuit the gap, and relatively higher conducting meanselectrically connecting said semi-conducting coating with at least oneof said electrodes.

RICHARD W. LAMPHERE.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,860,210 Spanner et al May 24,1932 1,897,587 Pirani Feb. 1.4, 1933 FOREIGN PATENTS Number Country Date30,745 Holland Sept. 15, 1933 118,827 Switzerland Feb. 1, 1927 417,192Great Britain Oct. 1, 1934 684,526 France Mar. 1KB, 1930

